Generally, turbomolecular pumps comprise two different kinds of pumping stages in cascade.
A first group of stages, called turbomolecular stages, are located in the suction or high vacuum portion of the pump; such stages are configured to work at very low pressures, in molecular flow regime.
A second group of stages, called molecular drag stages, are located in the exhaust or “low” portion of the pump; such stages are configured to work at higher pressure, up to viscous flow conditions.
It is known that gas pumping molecular drag stages in turbomolecular pumps are generally obtained from the interaction between stator channels formed into the pump body, and rotor discs mounted onto an integral for rotation with a rotary shaft driven into rotation by the pump motor. Corresponding tangential flow pumping channels, into which gas flows to be exhausted by the pump are defined between stator channels and rotor disks.
Pumping channels communicate with each other through corresponding inlet and outlet ports, axially arranged such that the outlet port in one stage is aligned with the inlet port in a second, downstream stage.
Between the inlet and outlet ports, the pumping channels are circumferentially interrupted by a block or obstruction, also called a “stripper”, generally formed in the stator channels, which provides for seal between inlet and outlet regions.
One of the problems encountered in developing a turbomolecular vacuum pump is the difficulty in exhausting gas to atmospheric pressure. When the pump cannot meet this requirement, generally a second pumping unit is provided at the outlet from the main pump, to allow attaining the desired pressure level.
Great efforts have been made in the past to obtain a turbomolecular pump capable of directly exhausting to atmospheric pressure, without need of providing a secondary pump.
More particularly, U.S. Pat. No. 5,456,575 assigned to Varian, Inc., discloses a pumping channel having a radial taper along its circumference, which taper allows increasing gas compression performance and extending the operating range of the turbomolecular pump.
Until now, generally only the possibility of varying the radial cross-section (or width) of the channel between the inlet and outlet ports has been considered, while leaving the axial cross-sectional size (or channel height) unchanged.
As known, the channel height is an essential parameter that significantly and differently affects important features, such as exhaust pressure and pumping rate of the pumping stage.
More particularly, in a molecular drag stage, the maximum exhaust pressure is inversely proportional to the square of the channel height. As a result, pumping channels are formed with the minimum possible height in order to obtain a high exhaust pressure.
On the contrary, pumping rate is directly proportional to the cross-sectional area of the channel inlet, hence to the channel height. This would lead to the contrary solution, i.e. to form pumping channels with a large height.
Thus, in the present turbomolecular pumps, in particular as far as the molecular drag stages are concerned, a trade-off must be found, by sacrificing the maximum exhaust pressure in favour of the pumping rate or vice versa.
It is a main object of the present invention to build a pumping stage for a turbomolecular pump allowing an optimum balance to be achieved between exhaust pressure and pumping rate.
It is another object of the present invention to build a molecular drag stage for a turbomolecular pump capable of exhausting gas to higher pressure than attainable by the conventional pumping stages.
It is a further object of the present invention to build a molecular drag pumping stage for a turbomolecular pump characterised by a lower energy dissipation in viscous flow than attainable by the conventional pumping stages.
The above and other objects are achieved by the pumping stage made in accordance with the invention, as claimed in the appended claims.